Rhodium catalysed hydroformylation of higher alkenes using amphiphilic ligands .2.

The performance of a new rhodium recycling system is described using a series of amphiphilic ligands: Ph(2)ArP (Ar = 3-hydroxyphenyl, 4-carboxyphenyl), Ph(n)Ar(3-n)P (Ar = 4-PhCH(2)X, X=NEt(2), NMePh, NPh(2); n = 1-2) and Ph(n)Ar(3-n)P (Ar = 3-pyridyl, 4-pyridyl; n = 1-2). The corresponding rhodium complexes are active in the hydroformylation of oct-1-ene and can be separated from the product aldehydes by acidic or basic extraction into water. After neutralisation of the aqueous phase the rhodium phosphine species can be extracted into a new batch of oct-1-ene, with toluene as organic solvent. The pH-depending distribution characteristics of the free ligands have been determined. PhAr(2)P with Ar = 4-PhCH(2)NEt(2) and Ar = 3-pyridyl were completely extracted from a solution in Et(2)O or toluene into a H2SO4-solution of pH 3 and 2, respectively, Recycling experiments with the former ligand demonstrated that the catalytically active rhodium hydrides decompose during the acidic extraction step. The structure of the recovered rhodium complexes could not be elucidated by NMR- and IR-analysis. Rhodium measurements by ICP-AES established that rhodium was quantitatively recycled. Pressurising the recovered rhodium and excess ligand to 20 bar syngas at 80 degrees C resulted in regeneration of the original, catalytically active species. A retention of catalytic activity up to 87% was achieved.